Haiti Earthquake 2010

Every year, earthquakes take the lives of thousands of people , and destroy property worth billions. The 2010 Haiti Earthquake killed over 1,50,000 people and destroyed entire cities and villages. Designing Earthquake Resistant Structures is indispensable. It is imperative that structures are designed to resist earthquake forces, in order to reduce the loss of life. The science of Earthquake Engineering and Structural Design has improved tremendously, and thus, today, we can design safe structures which can safely withstand earthquakes of reasonable magnitude.

Index of all posts on Earthquake Resistant Structures

1. Design Earthquake Resistant Buildings | Engineering Tips
2. Earthquakes and Natural Calamities
3. Types of Seismic Waves
4. Hazardous Effects of Earthquakes
5. Effect of Earthquakes on Structures
6. Building Stiffness and Flexibility | Earthquake Engineering
7. Inertial Forces in a Structure
8. Effects of Deformations in Structures
9. Horizontal and Vertical Shaking of a Structure
10. Flow of Inertia Forces to Foundations
11. How Earthquakes affect Reinforced Concrete Buildings
12. Building Planning | Earthquake Resistant Buildings
13. Earthquake Resistant Structures by Planning and Design Approach
14. Design Philosophy of Earthquake Resistant Designs
15. Building Construction Materials for Earthquake Resistance
16. Concept of Earthquake Resistant Engineering
17. Seismic Base Isolation Technique for Building Earthquake Resistance
18. Energy Dissipation Devices for Earthquake Resistant Building Design
19. Active Control Devices for Earthquake Resistance

Violent Ground Motion During Earthquakes

The seismic waves travel for great distances before finally losing most of their energy. At some time after their generation, these seismic waves will reach the earth’s surface, and set it in motion, which we surprisingly refer to as earthquake ground motion. When this earthquake ground motion occurs beneath a building and when it is strong enough, it sets the building in motion, starting with the buildings foundation, and transfers the motion throughout the rest of building in a very complex way. These motions in turn induce forces which can produce damage.

Haiti Earthquake Damage 2010

Real earthquake ground motion at a particular building site is vastly more complicated than the simple wave form. Here it’s useful to compare the surface of ground under an earthquake to the surface of a small body of water, like a pond. You can set the surface of a pond in motion – by throwing stones into it. The first few stones create a series of circular waves, which soon being to collide with one another. After a while, the collisions, which we term interference patterns, are being to predominate over the pattern of circular waves. Soon the entire surface of water is covered by ripples, and you can no longer make out the original wave forms. During an earthquake, the ground vibrates in a similar manner, as waves of different frequencies and amplitude interact with one another.

Building Frequency and Period

The characteristics of earthquake ground motions which have the greatest importance for buildings are the duration, amplitude (of displacement, velocity and acceleration) and frequency of ground motion.

Frequency

Frequency is defined as the number of complete cycles of vibration made by the wave per second.

Here we can consider a complete vibration to be the same as the distance between one crest of the wave and the next, in other words one full wavelength. Surface ground motion at the building site, then, is actually a complex superposition of vibration of different frequencies. We should also mention that at any given site some frequencies usually predominate.

The response of building to the ground motion is as complex as the ground motion itself, yet typically quite different. It also begins to vibrate in a complex manner, and because it is now a vibratory system, it also posses a frequency content. However, the buildings vibrations tend to center around one particular frequency, which is known as its natural or fundamental frequency. In general…

The shorter a building is, the higher its natural frequency. The taller the building is, the lower its natural frequency

Period

The natural period is the time it takes for the building to make one complete vibration.

The relationship between frequency F and period T is thus given as

T = 1 / F

This means that a short building with a high natural frequency also has a short natural period. Conversely, a very tall building with a low frequency has a long period.

Displacement of Building according to their Height & Stiffness

Ductility is the ability to undergo distortion or deformation without resulting in complete breakage or failure.  To see how ductility can improve a building’s performance during an earthquake, see the above figure.  In response to the ground motion, the rod bends but does not break.  (of course, metals in general are more ductile than materials such as stone, brick and concrete)  The ductility of a structure is in fact one of the most important factors affecting its earthquake performance.  One of the primary tasks of an engineer designing a building to be earthquake resistant is to ensure that the building will possess enough ductility to withstand the size and types of earthquakes it is likely to experience during its lifetime.

“If we have a poor configuration to start with, all the engineer can do is to provide a band-aid – improve a basically poor solution as best as he can. Conversely, if we start-off with a good configuration and reasonable framing system, even a poor engineer cannot harm its ultimate performance too much”.

Size of Buildings

Size of Buildings

In tall buildings with large weight-to-base size ratio the horizontal movement of the floors during ground shaking is large. In short but very long buildings, the damaging effects during earthquake shaking are many. And, in buildings with large plan area, the horizontal seismic forces can be excessive to be carried by columns and walls.

Horizontal Layout of Buildings

Buildings with simple geometry in plan perform well during strong earthquakes. Buildings with re-entrant corners, like U, V, H and + shaped in plan sustain significant damage. The bad effects of these interior corners in the plan of buildings are avoided by making the buildings in two parts by using a separation joint at the junction.

Vertical Layout of Buildings

Vertical Layout of Buildings

Earthquake forces developed at different floor levels in a building need to be brought down along the height to the ground by the shortest path, any deviation or discontinuity in this load transfer path results in poor performance of building. Buildings with vertical setbacks cause a sudden jump in earthquake forces at the level of discontinuity. Buildings that have fewer columns or walls in a particular storey or with unusually tall storey tend to damage or collapse which is initiated in that storey. Buildings on sloppy ground have unequal height columns along the slope, which causes twisting and damage in shorter columns that hang or float on beams have discontinuity in load transfer. Buildings in which RC walls do not go all the way to the ground but stop at upper levels get severely damaged

Adjacency of Buildings

Adjacency of Buildings

When two buildings are close to each other, they may pound on each other during strong shaking. When building heights do not match the roof of the shorter building may pound at the mid- height of the column of the taller one; this can be very dangerous.

Base Isolation Technique

The concept of base isolation is explained through an example building resting on frictionless rollers. When the ground shakes, the rollers freely roll, but the building above does not move. Thus, no force is transferred to the building due to the shaking of the ground; simply, the building does not experience the earthquake.

Now, if the same building is rested on the flexible pads that offer resistance against lateral movements (fig 1b), then some effect of the ground shaking will be transferred to the building above. If the flexible pads are properly chosen, the forces induced by ground shaking can be a few times smaller than that experienced by the building built directly on ground, namely a fixed base building (fig 1c). The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings. The main feature of the base isolation technology is that it introduces flexibility in the structure.

As a result, a robust medium-rise masonry or reinforced concrete building becomes extremely flexible. The isolators are often designed, to absorb energy and thus add damping to the system. This helps in further reducing the seismic response of the building. Many of the base isolators look like large rubber pads, although there are other types that are based on sliding of one part of the building relative to other. Also, base isolation is not suitable for all buildings. Mostly low to medium rise buildings rested on hard soil underneath; high-rise buildings or buildings rested on soft soil are not suitable for base isolation.

Concept of Base Isolation

Lead-rubber bearings are the frequently-used types of base isolation bearings. A lead rubber bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of the solid lead “plug”. On top and bottom, the bearing is fitted with steel plates which are used to attach the bearing to the building and foundation. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.

How it Works

To get a basic idea of how base isolation works, first examine the above diagram. This shows an earthquake acting on base isolated building and a conventional, fixed-base, building. As a result of an earthquake, the ground beneath each building begins to move. . Each building responds with movement which tends towards the right. The buildings displacement in the direction opposite the ground motion is actually due to inertia. The inertia forces acting on a building are the most important of all those generated during an earthquake.

In addition to displacing towards right, the un-isolated building is also shown to be changing its shape from a rectangle to a parallelogram. We say that the building is deforming. The primary cause of earthquake damage to buildings is the deformation which the building undergoes as a result of the inertial forces upon it.

Response of Base Isolated Buildings

The base-isolated building retains its original, rectangular shape. The base isolated building itself escapes the deformation and damage-which implies that the inertial forces acting on the base isolated building have been reduced. Experiments and observations of base-isolated buildings in earthquakes to as little as ¼ of the acceleration of comparable fixed-base buildings.

Acceleration is decreased because the base isolation system lengthens a buildings period of vibration, the time it takes for a building to rock back and forth and then back again. And in general, structures with longer periods of vibration tend to reduce acceleration, while those with shorter periods tend to increase or amplify acceleration.

Spherical Sliding Base Isolation

Spherical Sliding Base Isolation

Spherical sliding isolation systems are another type of base isolation. The building is supported by bearing pads that have a curved surface and low friction. During an earthquake the building is free to slide on the bearings. Since the bearings have a curved surface, the building slides both horizontally and vertically. The forces needed to move the building upwards limits the horizontal or lateral forces which would otherwise cause building deformations. Also by adjusting the radius of the bearings curved surface, this property can be used to design bearings that also lengthen the buildings period of vibration